FIG. 1 shows a conventional fluidic microlens 101. As shown, the microlens comprises a substrate 110. The substrate includes a ground electrode 121 between first and second control electrodes 123a-b. The ground electrode is coupled to ground reference voltage while the control electrodes are coupled to variable voltage sources. Dielectric and coating layers 130 and 135 are disposed over the electrodes. The coating layer comprises a hydrophobic material, such as polytetrafluoroethylene (e.g., Teflon). The dielectric and coating layers are patterned to create a window 139, exposing the ground electrode.
A drop of conductive liquid 140 is disposed on the coating layer. The drop serves as the microlens. The drop contacts the surface and ground electrode. In an inactive stage, (no voltage applied to the control electrodes), the drop takes on a first shape, as designated by the solid line 141. This shape depends on the size of the drop and the surface energy of the hydrophobic coating layer. When a voltage is applied to the control electrodes, the electric potential from the voltage causes the hydrophobic coating layer to become hydrophilic (e.g., change in wettability), thereby changing the curvature of the drop, as designated by the dotted line 142. The change in curvature changes the focal length. Changing the wettability of the coating layer using an electric field to change the focal length of the fluidic microlens is known as “electrowetting”.
However, various problems are associated with electrowetting-controlled fluidic microlenses, limiting their performance. Such problems, for example, include liquid evaporation, contact angle hysteresis, low focal length tunability and high driving voltages. Additionally, the surface energy of the hydrophobic layer creates discrete rapid jumps in curvature change of the microlens instead of a smooth continuous one. This is referred to as the stick-slip behaviour.
Therefore, in view of the foregoing discussion, it is desirable to provide an improved microlens which avoids the problems associated with conventional microlens.